The present invention pertains to certain rubber-modified monovinylidene aromatic polymer compositions which exhibit a beneficial combination of physical characteristics.
It is well known in the art that various relatively rigid and/or brittle interpolymers of monovinylidene aromatic monomers with ethylenically unsaturated nitrile monomers can be made more impact resistant by the inclusion of amounts of various types of elastomeric materials, such as various rubbers, into a matrix or continuous phase of said interpolymers. The elastomeric materials usually are in the form of discrete particles, such particles having amounts of the matrix interpolymer, or an inter- or homopolymer similar thereto, graft-polymerized thereto. These types of rubber-modified, impact-resistant polymeric compositions are commonly known and referred to as graft copolymers or polyblends. Among the best known of these types of compositions are the ABS or ABS-type compositions. Compositionally, ABS or ABS-type compositions generally comprise a combination of an elastomer, usually containing polymerized butadiene, with a rigid interpolymer of a monovinylidene aromatic monomer and an ethylenically unsaturated nitrile monomer. Structurally, ABS or ABS-type compositions usually consist of the rigid, matrix or continuous phase having dispersed therein particles of the elastomer, such particles usually having grafted thereto amounts of the rigid interpolymer or a similar inter- or homopolymer.
It is also well known in the art that the physical properties of these types of compositions are greatly affected by the relative amounts of elastomer particles having different sizes and particle structures, as well as the structures of the individual rubber particles (i.e., rubber particle morphology). There are well-known advantages and disadvantages accompanying the use of either of the two main rubber-particle types in rubber-modified polymer compositions. It is generally believed that grafted rubber particles containing occlusions of matrix polymer therein, this being one of the two main rubber-particle types, provide more impact resistance than the same amount of rubber in the form of similarly grafted, solid rubber particles, the latter being the other main particle type. The grafted, occlusion-containing rubber particles are usually formed and grafted in a mass-type or mass-suspension-type polymerization process where a previously-produced rubber is dissolved in one or more polymerizable monomers with optional diluents, which monomers are thereafter polymerized. Occlusion containing particles, produced in such mass, mass-solution or mass-suspension processes or variations of these processes are hereafter referred to a "mass particles." It is difficult, however, using available types of rubber and mass process equipment to produce groups of mass particles having volume average diameters less than 0.5.mu..
The other main type of rubber particle morphology (i.e., the above-mentioned "solid" or non-occluded grafted rubber particle) is usually achieved via emulsion polymerization of the rubber in the form of an aqueous latex. After the rubber is made, monomers which are polymerizable and graftable (e.g., styrene and acrylonitrile) are usually added to the rubber-containing latex and polymerized to form the graft portion as well as amounts of matrix polymer. The non-occluded type of rubber particles, produced via an emulsion polymerization process, are hereinafter referred to as "emulsion particles." When these emulsion particles have been grafted with a different, relatively rigid polymer, but still have a high rubber concentration, at least about 30 weight percent or so, these compositions are very suitable for blending with additional amounts of the same or different rigid polymer, optionally containing additional amounts of rubber, to achieve desired rubber contents in the resultant compositions. Such blendable intermediates are often referred to as "grafted rubber concentrates" and can be used to produce a wide variety of rubber-modified polymer compositions.
It is further known that the heat distortion temperature or softening point of ABS or ABS-type composition can be raised by incorporating into the compositions materials such as N-phenylmaleimide, .alpha.-methyl styrene, and copolymers of styrene with maleic anhydride. However, the incorporation of these materials into ABS or ABS-type compositions usually is accompanied by some decrease in other physical properties. For example, the use of styrene-maleic anhydride can result in uncontrolled cross-linking at temperatures greater than 230.degree. C., resulting in unpredictable decreases in impact and melt flow rate properties. Similarly, the use of .alpha.-methyl styrene as a comonomer can result in a composition which is difficult to process. Specifically, the use of .alpha.-methyl styrene can lower the ceiling temperature of the resulting composition, the result being depolymerization and reduced physical properties. Accordingly, the use of certain materials may improve the heat distortion temperature of an ABS or ABS-type composition, but generally at the expense of other physical properties. Thus, a long-standing problem is the production of an ABS or ABS-type composition having a high heat distortion temperature while also exhibiting impact and tensile strength properties.
U.S. Pat. No. 4,567,233 discloses a rubber-modified styrenic resin composition comprising up to four components. One component is a graft copolymer comprising a specific matrix resin and emulsion rubber particle having an average particle size of from 0.1 to 0.5.mu.. Another component is a graft copolymer comprising a specific matrix resin and a mass or mass-suspension rubber particle having an average particle size of from 0.7 to 4.mu.. The amount of the small particle component must be from 50 to 97 percent of the total weight of the two rubber-containing components. A third component includes a copolymer of a vinyl aromatic compound, a maleimide compound, and optionally a copolymerizable vinyl compound. An optional fourth component is a polymer of a vinyl aromatic compound and an unsaturated nitrile compound. Said patent discloses that such compositions have good heat stability and falling dart impact properties. However, the problem of tensile strength is not discussed.
It is known that the use of small rubber particles improves tensile properties for a given rubber content, but sacrifices impact properties. Further, the use of emulsion particles, which generally are smaller, results in degradation of thermal properties, especially the Vicat softening point, compared to the use of mass particles. This is because higher rubber levels are required in an emulsion ABS to attain a given impact value. The Vicat softening point temperature is further reduced by the presence of residuals from the emulsion process. Thus, while the use of emulsion particles in ABS and ABS-type polymers is common for the improvement of certain properties, such use is detrimental to the thermal and tensile strength properties of these resins.
Since the foregoing problem is fairly commonly encountered within the plastics molding industry, it would be highly desirable if there could be provided rubber-modified molding compositions having excellent tensile properties and, particularly, if such could be accomplished without attendant sacrifices in other important properties such as impact strength and heat resistance.